In comparison with modem day digital methods, a number of disadvantages are associated with conventional analog TV transmission. Mention is made here, in particular, of the fact that analog transmission is highly susceptible to interference. The extent of the interference effects ranges from shadowing, distortions and partial image drop-outs, to total image drop-outs. Multipath propagation effects, in particular, contribute to this interference.
This so-called multipath propagation stems for the most part from a specific signal component reaching the receiver via a direct path, while other signal components, due, for example, to reflections off of specific terrain features, such as steeply rising mountain slopes, reach the receiver via an indirect path and thus, with a slight delay as compared to the directly received signal. The superposition of these two signals results in a clearly degraded reception and, respectively, reproduction quality.
In a vehicle that is underway, the effects of multipath propagation are even greater due to the constantly changing topography of the surroundings. Thus, an incoming signal in an urban area behaves completely differently than in a rural area. In the city, the differences in the signal propagation times are substantially smaller than in rural or mountainous environments. This can mostly be attributed to the fact that, in the city, the signal is reflected off of neighboring buildings, whereas, in rural surroundings, the signal is more likely to be reflected off of distant mountains.
Thus, the need exists to find ways to reduce or even to eliminate such interference. To this end, in recent years, advances in technology are making new approaches possible. Thus, in the meantime, it has become possible to sample broadband analog signals, such as the TV signal, for example, in order to subsequently further process it digitally. This had not been possible under known methods heretofore due to the low computing power of the commonly used semiconductor components.
A plurality of approaches are known under the related art for this type of further processing for suppressing multipath propagations. In the special case of the analog TV, it is conceivable, for example, to use the line synchronization signal in order to make use of a channel estimation for purposes of signal correction. This can be undertaken, for example, by using filters whose characteristic is the inverse of the impulse response for purposes of signal correction. In this context, however, the filter regularly experiences stability problems, so that this approach does not appear to be very promising.
In addition, a method is presented in the German Patent DE 36 87 164 T3, which provides for the filter coefficients of a digital filter to be calculated exclusively by using correlation operations to compensate for multipath propagation.
However, the method described in the mentioned publication does not require that a reference value be obtained from a signal that is, in particular, not distorted (i.e., not interfered or noise-corrupted) by multipath propagations.
Moreover, the optimization can only be accomplished on the basis of the synchronization pulses which are not continuously present in the signal, so that the quality of the signal conditioning is substantially degraded.